A lithium ion secondary battery is a small, lightweight battery with a large capacity and has been widely used as a power source for portable instruments since its advent in 1991. Recently, as a result of rapid developments in the electronic, communication and computer industries, a camcorder, a mobile phone and a notebook computer have been remarkably developed. Therefore, as a power source for driving these info-communication instruments, a lithium ion secondary battery is in greater demand. In particular, a lot of recent studies on a power source for electric vehicles by hybridization of an internal combustion engine and the lithium secondary battery are actively undertaking in many countries including USA, Japan and Europe. The current commercially available small-sized lithium ion secondary battery uses LiCoO2 as a cathode and carbon as an anode, respectively. As a cathode material that is actively being studied and developed, mention may be made of LiNiO2, LiNi1−xCoxO2 and LiMn 2O4. LiCoO2 is an excellent material having stable charge/discharge characteristics, superior electron conductivity, high thermal stability and flat discharge voltage characteristics, but Co is in shortage of ore reserve, expensive and, further, toxic to humans, and therefore, there remains a need for developing alternative electrode active materials. Since LiMn2O4 having a spinel structure has a theoretical capacity of about 148 mAh/g, which is small when compared to other materials, and a three-dimension al(3-D) tunnel structure, it exhibits a large diffusion resistance upon insertion/release of lithium ions, thereby has a relatively low diffusion coefficient, as compared to LiCoO2 and LiNiO2, having a two-dimensional(2-D) structure, and has poor cyclability to a Jahn-Teller effect. Particularly, it exhibits poor characteristics at temperatures of more than 55° C., compared to LiCoO2 and thereby is not widely used in practical battery applications at present. Therefore, a great deal of studies on materials having a layered crystal structure have been undertaken as a material capable of overcoming the problems as mentioned above. LiNiO2 having a layered structure like LiCoO2 shows large discharge capacity, but has disadvantages such as difficulty of material synthesis, rapid reduction of capacity due to changes in a crystal structure accompanied by charge/discharge cycles and a problem associated with thermal stability, thereby failing to be commercialized.
As an attempt to stabilize the crystal structure of LiNiO2, there is known a technique realizing improved charge/discharge characteristics and thermal stability by replacing portions of Ni sites with Co, Al, Ti or Mn. In this regard, a number of techniques are known which relate to the preparation of a Li—Ni—Mn based composite oxide in which portions of Ni sites were replaced with Mn, or a Li—Ni—Mn—Co based composite oxide in which portions of Ni sites were replaced with Mn and Co. For example, U.S. Pat. No. 5,264,201 discloses a solid phase method involving mixing hydroxides or oxides of Ni and Mn with an excess amount of lithium hydroxide, or a synthetic method involving slurrifying oxides of Ni and Mn in an aqueous saturated lithium hydroxide solution, drying the slurry thus obtained in vacuo under a reduced pressure and calcining to obtain a material of formula LixNi2−x−yMnyO2 wherein x is between 0.8 and 1.0,and y is equal to or less than 0.2. Further, U.S. Pat. No. 5,626,635 discloses a technique relating to a Li—Mn—Ni—O composition, U.S. Pat. No. 6,040,090 and Japanese Patent Publication Laid-Open No. Hei 8-213015 disclose a technique relating to a Li—Mn—Ni—Co—O composition. In addition, Japanese Patent Publication Laid-Open No. Hei 8-171910 proposes a method for preparing a cathode active material of the formula LiNixMn1−xO2 wherein x is between 0.7 and 0.95,comprising mixing an alkaline solution with an aqueous mixed solution containing manganese and nickel, co-precipitating manganese and nickel, mixing the resulting co-precipitated compounds and lithium hydroxide and calcining them. Recently, Japanese Patent Application No. 2000-227858 discloses a new type of the cathode active material by homogeneously dispersing nickel and manganese compounds at an atomic level to prepare a solid solution, unlike the concept of partially replacing LiNiO2 or LiMnO2 with a transition metal.
Further, recently, Japanese Patent Publication Laid-Open Nos. 2003-238165, 2003-203633, 2003-242976A, 2003-197256A, 2003-86182, 2003-68299 and 2003-59490 disclose a process for preparing a high capacity cathode active material having improved charge/discharge reversibility and thermal stability, comprising dissolving nickel and manganese salts, or nickel, manganese and cobalt salts in an aqueous solution, simultaneously adding an alkaline solution to a reactor in order to obtain a metal hydroxide or oxide as a precursor material while purging with a reducing agent or inert gas, mixing the precursor material with lithium hydroxide followed by calcination.
As portable electronic instruments realize high performance, realization of high capacity and large current of a lithium secondary battery is also in great demand. In order to accomplish this purpose, it is necessary to develop the cathode active material which is capable of increasing an amount of the cathode active material by increasing the packing density of the cathode active material or reducing an amount of a conductive material incorporated into a cathode plate while improving electron conductivity and ion conductivity of the cathode active material and exhibiting no changes in the crystal structure due to charge/discharge cycles. In order to achieve this, there has been made a variety of studies. As one method for this purpose, there have been made studies of increasing charge efficiency by forming the cathode active material in the form of a sphere, increasing a contact area between electrode active materials by improving charge efficiency and thus improving conductivity, and increasing the amount of the active materials by reducing an amount of a conductive material in a composite cathode. For example, Japanese Patent Publication Laid-Open Nos. 2003-86182, 2003-68299 and 2003-59490 disclose a process for preparing a lithium composite hydroxide by mixing and calcining nickel hydroxide, cobalt hydroxide, manganese hydroxide and lithium hydroxide in the form of a sphere or oval thereby aggregating primary particles to form secondary particles. However, in this compound, there are many voids between the primary particles and thus the tap density is relatively low when compared to a lithium cobalt oxide, resulting in a decreased amount of the chargeable active material leading to difficulty in realizing a high capacity.
Recently, LiNi1−xCoxO2 wherein X is equal to or greater than 0.7,which receives a great deal of attention as a high capacity material, exhibits excellent charge/discharge characteristics and high capacity characteristics of more than 180 mAh/g, but is limited in its practical use in a battery due to instability of Ni3+ or Ni4+ when charging.
As a material having the layered crystal structure capable of substituting LiCoO2, to which a great deal of attention is directed, mention may be made of Li[Ni1/2Mn1/2]O2 and Li[Ni1/3Co1/3Mn1/3]O containing a 1:1 mixture of Ni—Mn and Ni—Co—Mn, respectively. These materials exhibit relatively low costs, high capacity and excellent thermal stability compared to LiCoO2. However, such materials have the relatively low electron conductivity compared to LiCoO2, and thus the high-rate and low temperature characteristics are poor, and it is failed to improve the battery energy density due to a low tap density, in spite of the high capacity. In particular, in the case of Li[Ni1/2Mn1/2]O2, it has a very low electron conductivity, thus making it difficult to realize a practical use (J. of Power Sources, 112(2002) 41-48) Particularly, these materials exhibit inferior high output power characteristics as compared to LiCoO2 or LiMn2O4, for use in electric vehicles as a hybrid power source.
A general method for preparing Li[Ni1/2Mn1/2]O2 and Li[Ni1/3Co1/3Mn1/3]O2 is performed by simultaneously precipitating two or three elements in an aqueous solution using a neutralization reaction, so as to a precursor in the form of hydroxide or oxide, mixing the precursor thus obtained with lithium hydroxide and then calcining them. Unlike a conventional co-precipitation reaction, the co-precipitated particles containing manganese are usually irregular plate-like and have a tap density that is 50% when compared to that of nickel or cobalt. For example, Japanese Patent Publication Laid-Open 2002-201028 uses a conventional reactor by way of an inert precipitation method, and the resulting precipitated particles have very broad distribution of particle sizes and shapes of the primary particles are different from each other. Further, International Application PCT WO 02/078105A1,filed by Matsushita Electric Industrial Co., Ltd., proposes a gravity-precipitation type of a reactor using gravity of the precipitated materials in order to obtain uniform particles, but suffers from disadvantages such as being costy due to a need for increasing pressure of a circulation pump when scaling-up, and operation difficulty due to complicated process caused by addition of additional equipments. In order to resolve these problems, a method of inducing formation of spherical particles using high-speed rotation has been conventionally employed. However, even though the conventional reactor is a continuous reactor by high speed rotation, application of such method presents disadvantages such as irregular changes in volume of reactant materials, volume expansions due to violent waves and a Ni—Co—Mn composite hydroxide having an irregular plate-like shape of particles, thus lowering the tap density.